1. Field of the Invention
The present invention relates to analog-to-digital converters and, more particularly, to converters for use with analog signals having very large dynamic ranges leading to use of floating point number digital representations.
Many analog signals provided from sensors used to measure various phenomena have very large amplitude differences between various portions of them because of the great differences in energies between various aspects of the phenomena being measured. Events to be measured often are such that the sensors yield corresponding signals with the strongest being millions of times greater in amplitude than are the weakest. As an example, the measuring of seismic events can yield signals both for ground vibrations caused by nearby powerful earthquakes and human footsteps hundreds of yards away.
Those corresponding signals obtained from sensors for measuring such events are provided as analog signals, but the usual desire is to have these measurements recorded as a series of digits forming numbers that indicate signal magnitude at selected points. Such digital representations of selected sample points of the analog signal are conveniently manipulated by digital computers in efforts to glean information from the signals. This desire leads to the need for an analog-to-digital converter arrangement which can take such wide ranging analog signals and provide suitable numerical equivalents, each representing subranges of the analog signal. The best converters for direct conversions of analog signals provided thereto that are typically available for this purpose (best in providing representation numbers with as many effective digits as possible for resolution) provide binary numbers of sixteen digits as a result of the conversion. This capability means that the number of different analog signal levels which can be resolved will be equal to two to the sixteenth power or 65,536. Such a converter can convert an analog signal which has its greatest amplitude portions being no more than 65,536 times the amplitude of the smallest signal for which the converter can provide a valid numerical equivalent. Obviously, such a converter is an inadequate means to provide numerical equivalents for analog signals having significant portions thereof which are a million or more times greater or smaller than other portions thereof.
The usual solution to this problem is to use such a direct converting analog-to-digital converter able to directly convert the greater amplitude portions of analog signals and to also provide an analog signal amplifier arrangement for smaller amplitude portions. This amplifier can have its amplification adjusted until its output signal is of a size sufficient for conversion by this analog-to-digital converter. The numerical result from the converter along with the gain used in connection with the analog signal amplifier are together used as a floating point number representing the analog signal value presented to the amplifier arrangement.
The numerical result from such a conversion arrangement is a scaled number, that is, a number scaled in size so as to be in a range which the converter can provide at its output, typically to be a number in the upper half of the numbers which the converter can provide to give a good resolution capability. The gain of the analog signal amplifier is related to a scaling number which must be used to multiply the scaled number, also called the mantissa, to give a numerical value substantially equivalent to that of the analog signal value selected to be converted. Since the smallest number representation that the converter can supply when the amplifier gain is at its maximum is also the smallest number that the converter arrangement can supply, this is defined as the system zero value. Thus, the difference between a lesser gain value used in numerical representations of other analog signal values and the amplifier maximum gain is a direct measure of the scaling number. This sort of a solution can give a capability of providing for the conversion of analog signal values over a range many times that which could be converted using the direct converting analog-to-digital converter alone.
Nevertheless, such a solution is difficult at best. Usually there are several amplifiers connected in series or parallel, or a varying gain amplifier will be used. Determining which one to use to effect a conversion, or when gain to use, considerably slows the conversion rate possible and requires expensive equipment or the risk of error of a user operating manually, or both. Such amplifiers will also each suffer from certain offset error values and gain errors which are then reflected in the analog signal as incorporated error values added to, or multiplying, the actual signal values provided by the sensor. Furthermore, the direct converting analog-to-digital converter itself will introduce errors of a similar sort. Attempts to overcome these errors will often require the use of many expensive precision components, the use of careful operator adjustments, or various rather extensive feedback arrangements, or some combination of these measures. Thus, a converter system avoiding some or all of these difficulties is desirable.